In recent years, multilevel modulation is being applied more and more for the purpose of improving the frequency utilization efficiency in addition to giving an optical communication system a large capacity. However, it is a known fact that raising the multilevel degree in order to increase the transmission capacity decreases phase noise tolerance proportionately (see, for example, Non Patent Literature 1).
A technology of inserting a pilot signal has been proposed in order to guarantee transmission performance to deal with phase slipping due to phase noise that is caused by multilevel modulation transmission (see, for example, Non Patent Literature 2).
Inserting a pilot signal, however, increases the degree of redundancy depending on the frequency of pilot insertion, and the accompanying rise in the operating speed of an electric circuit leads to a problem of difficulty in dealing with high transmission rate.
As another known technology to deal with the phase slipping, there is known to use differential encoding. This technology gives tolerance for changes in absolute phase by providing information about a transition between consecutive signal points on a phase space of two I/Q signals that are orthogonal to each other. This does not require the insertion of an additional signal, which means that there is no accompanying rise in signal operating speed, and therefore has an advantage of excellent suitability for high-speed signal transmission.
In a known technology related to multilevel-signal demodulation processing, a more significant bit of a received signal is used to determine a quadrant of the signal's coordinates, whereas likelihood generating processing is performed with respect to a less significant bit of the received signal independently of the more significant bit (see, for example, Patent Literature 1).
There is also known a technology of receiving a modulated signal that has been encoded differentially and generating likelihood with respect to the received signal (see, for example, Patent Literature 2, Patent Literature 3, and Patent Literature 4).
In still another known technology, likelihood generating processing is performed separately with respect to a more significant bit and less significant bit of a received quadrature amplitude modulation (QAM) signal vector (see, for example, Patent Literature 5, Patent Literature 6, and Patent literature 7).
In a known technology for a method of differentially encoding two bits on the most significant bit (MSB) side which are associated with quadrant identification, and encoding bits on the least significant bit (LSB) side by Gray encoding, decoding processing combined with symbol rotation is performed on the LSB-side bits (see, for example, Patent Literature 8).
A digital coherent transmission technology that uses digital signal processing is drawing attention as a technology for accomplishing a transmission rate of 100 Gbps or higher.
With this technology, amplitude information obtained with the use of an analog-to-digital converter (ADC) can be applied to the processing of a received signal, which makes it easy to employ, in combination, an error correction technology that uses soft decision, and improves reception performance.